While electroceramics are used in a host of commercially important applications, the achievable performance of multi-element ceramic compositions is frequently impaired by difficulties in achieving desired chemical and phase compositions. For describing the nature of these problems, it is useful to consider the specific cases of lead metaniobate (Pb(NbO.sub.3).sub.2) and lead magnesium niobate (PbMg.sub.1/3 Nb.sub.2/3 O.sub.3) phases, which could have increased applicability as high performance ferroelectrics if the materials quality could be improved, especially using inexpensive processing methods.
Goodman (J. Am. Ceram. Soc., vol. 36, pp. 368-372, 1952, U.S. Pat. No. 2,729,757) first prepared lead metaniobate by reacting a mixture of PbSO.sub.4 and Nb.sub.2 O.sub.5. The PbSO.sub.4 decomposes to form highly reactive PbO, which reacts with the Nb.sub.2 O.sub.5. The resulting reaction product is a mixture of orthorhombic lead metaniobate and an undesired pyrochlore phase Pb.sub.2 Nb.sub.2 O.sub.7. Also, calcinating a stoichiometric mixture of metal oxides does not provide high quality lead metaniobate powder. For example, sintering a compacted disk made from a mixture of PbO and Nb.sub.2 O.sub.5 fine powders (at about 950.degree. C. under oxygen flow for 15 days) is associated with the volatilization of lead oxide, which makes it difficult to control chemical composition and phase purity (see Rivolier et al., Eur. J. Solid State Inorg. Chem., vol. 32, pp. 251-262, 1995; U.S. Pat. No. 2.729,575). In addition, U.S. Pat. No. 4,234,558 to Arendt and Rosolowski describes the preparation of this ceramic powder by the molten salt method. However, the resulting powder contains not only an undesired phase, but also contaminants from the salts, which degrade the performance of the ceramic as a ferroelectric. Yamaguchi and Mukaida (J. Mat. Sci. Lett., Vol. 9, pp. 556-558, 1990) prepared rhombohedral lead metaniobate by the co-precipitation method using an alkoxy precursor route. Boulmaaz et al. (J. Mat. Sci. Vol. 7, pp. 2053-2061, 1997) later found that similar methods produced the undesired PbNb.sub.4 O.sub.11 as an impurity phase. Matra et al. (J. Thermal Analysis, Vol. 44, pp. 431-440, 1995) attempted to produce lead metaniobate powder by a co-precipitation method starting from dissolving inorganic salts in acidified water, but no details concerning phase purity or phase composition were provided.
Lead magnesium niobate (PMN) is a perovskite that is a relaxor ferroelectric. Like lead metaniobate, it is very difficult to synthesize as a pure phase because conventional solid-state reaction methods result in the formation of a very stable pyrochlore phase. Although some researchers have reported the successful synthesis of this ceramic, such synthesis involves repeated calcinations at high temperatures and long sintering times. Consequently, it is difficult to control the stoichiometry of the ceramic because of the loss of PbO (S. J. Sang et al., Ferroelectrics, Vol. 27, pp. 31-35, 1980). Swartz and Shrout (Mat. Res. Bull., Vol. 17, pp. 1245-1250, 1982) have described a process for minimizing the formation of the pyrochlore phase. For this process, which involves sintering a pellet compacted from a stoichiometric mixture of the columbite phase (MgNb.sub.2 O.sub.6) and lead oxide, these researchers reported forming about 99% perovskite. This is called the columbite method. The addition of a few percent excess of MgO is reported to further decrease the amount of the pyrochlore phase. The addition of excess of PbO is also reported to have the same effect (LeJeune, M. and Boilot, J. P., Am. Ceram. Soc. Bull., Vol. 64, pp. 679-684, 1985). Application of sol-gel methods for this ceramic provides controversial results. Chaput et al. (J. Am. Ceram. Soc., Vol. 72, pp. 1335-1357, 1989) reported that a pure perovskite phase could be obtained, but Fukui et al. (J. Ceram. Soc. Japan., Vol. 102, pp. 393-396, 1994) later found a significant amount of the pyrochlore phase. Currently, the PMN related electroceramic is prepared mainly by the columbite method.
Mechanochemical synthesis is a solid-state synthesis technique in which the synthesis process is initiated or facilitated by a mechanical process, such as grinding. This technique has been rarely used in preparation of ferroelectrics, since mechanochemical reaction between metal oxides usually proceeds to less than 50% completion even under high-energy milling condition.
Streleskii and Borunova have mechanochemically synthesized lead metaniobate, PbNb.sub.2 O.sub.6, and other lead-oxide-related ceramics (Proceedings of the First International Conference on Mechanochemistry, Vol. 1, pp. 98-101, 1993). In another effort to use mechanochemistry, Komasubara et al. (J. Amer. Ceram. Soc., Vol. 77, pp. 278-282, 1994) prepared lead titanate, PbTiO.sub.3, by milling a mixture of dried lead and titanium sol-gels. There are indications that the milling induced some degree of mechanochemical reactions between the sol-gels. Using a similar approach, Hamada et al. (J. Mat. Sci. Lett., vol. 15, pp. 603-605 1996) prepared magnesium titanate, MgTiO.sub.3, precursors by milling (1) TiO.sub.2 and Mg(OH).sub.2 mixtures and (2) TiO.sub.2 and MgO sol-gel mixtures. These researchers found that mechanical milling could affect the crystallization behavior of the products during sintering. Balaz et al. (J. Mat. Sci., vol. 29, pp. 4847-4851, 1994) and Abe and Suzuki (Mat. Sci. Forum, vols. 225-227, pp. 563-568, 1996) employed mechanochemical phenomena in the preparation of barium titanate, BaTiO.sub.3, fine powder. In this work, Balaz used mechanical milling to mechanically activate a mixture of BaCO.sub.3, TiO.sub.2 and PbO. Such activated mixture offered a BaTiO.sub.3 powder having a narrow particle size distribution after thermal treatment at 1100.degree. C. Abe et al. used high energy planetary ball milling to initiate a mechanochemical reaction between TiO.sub.2 and Ba(OH).sub.2. A poorly crystalline BaTiO.sub.3 was obtained. Other researchers have attempted the preparation of barium hexaferrite powders using a similar approach (Solid State lonics, Vol. 101-103, pp. 103-109, 1997). Baek et al. synthesized lead magnesium niobate-lead titanate (PMN-PT) without pyrochlore phase by sintering a stoichiometric mixture of PbO, TiO.sub.2, Nb.sub.2 O.sub.3 and Mg(OH).sub.2 which had been treated by mechanical milling (Mat. Sci. Forum, Vols. 235-238, pp. 115-120, 1997). Such milling could potentially cause reaction between Nb.sub.2 O.sub.5 and Mg(OH).sub.2 to form the Columbite phase MgNb.sub.2 O.sub.6, which might be why they did not find any pyrochlore phase. Awano and Takagi (J. Ceram. Soc. Japan, Vol. 101, pp. 124-128, 1993) tried the mechanical grinding of a gel powder prepared from La.sub.2 O.sub.3, Pb(NO.sub.3).sub.2, and TiCl.sub.4 --Zr-oxychloride to promote mechanochemical reactions for the synthesis of lead lanthanum zirconate titanate, PLZT [(Pb.sub.0.92 La.sub.0.08)(Zr.sub.0.65 Ti.sub.0.35).sub.0.98 O.sub.3 ]. Isobe et al. also employed ball milling of a powdered mixture of two gels (prepared from mixing ZrO.sub.2 and TiO.sub.2 sols) to produce a precursor for ZrTiO.sub.4 ceramics (Mat. Res. Symp. Proc.: Better Ceramics through Chemistry VI, pp.273-277, 1994).
In the prior art, compositional heterogeneity, unexpected by-products, incomplete reactions, contaminates, and strong aggregation are disadvantages which restrict the applicability of mechanochemical processes for advanced ceramic preparation (Abe and Narita, Solid State Ionics, Vols. 101-103, pp. 103-109, 1997). Several methods have been utilized by the prior art in efforts to improve this technology, which include hydrothermal enhancement of mechanochemical synthesis (Kosova et al., Solid State Ionics, Vols. 101-103, pp. 53-58, 1997), the use of high energy planetary mills (Abe and Narita, Solid State Ionics, Vols. 101-103, pp. 103-109, 1997), and hydroxide-group-modified mechanochemical interaction, which introduces OH groups to adjust surface acidity or basicity (Senna et al., Mat. Sci. Forum, vols. 225-227, pp. 521-526, 1996). The above efforts in process improvement are based on the physical modification of the presently existing mechanochemical synthesis. No significant improvements were achieved, especially with respect to improving compositional homogeneity, degree of the reaction, and the elimination of by-products.